This invention relates to a feed and cutting device for dividing a continuous web into portions, and more precisely a device of the aforesaid type able to operate with great reliability even at the very high operating speeds of the most modern wrapping and packaging machines.
A device of this type is particularly suitable in those cases in which, for reasons of accuracy, the cutting operation has to be preceded by a web adjustment operation. This is the case for example when removing, from a continuous web, portions or sheets comprising a printed motif on their surface.
In this respect, it is well known that the distance between the centres of such printed motifs on a continuous web is not exactly constant for various reasons deriving from the actual printing process, from the variation in the tension to which the web is subjected during its unwinding, and from variations in ambient conditions which can cause it to contract or elongate.
For these reasons, when a web printed on one or both of its faces has to be divided into portions, each cutting operation is of necessity preceded by an adjustment operation.
The final result is portions having a length which is not precisely constant, and which carry on their surface printed motifs which are perfectly centred.
In known devices, feed means drive the web with an intermittent movement along a plane known as the feed plane.
The web is adjusted relative to the cutting means when the feed means are in their rest or halt stage.
The means proposed for this purpose can either be electrical or mechanical, and operate in combination with reference marks (colour marks, holes, slots) applied to the web during the printing, at the same distance apart as the printed motifs.
After this adjustment, the operational cycle of these devices is concluded by a portion or sheet being separated, this being done by cutting means disposed downstream of said feed means.
Upstream of and in proximity to said feed means, guide means for the web are normally provided along the feed plane, these being constituted for example by fixed means or by a pair of idle rollers. The purpose of these rollers is to ensure that the web is controlled, in particular when the feed means are in their inactive or halt stage, and during the adjustment operation.
The drawbacks of such devices of known type include the cyclic variations in the tension to which the web is subjected due to the alternation of the active and the inactive or halt stages of the feed means.
Beyond determined operational speeds of the device, and particularly when using delicate materials, these variations in tension can cause tearing of the web. Such tearing is greatly facilitated in the case of webs in which transverse slots have been formed to act as the reference marks for mechanical adjustment means.
In this respect, the transverse slots represent lines of weakness at which preferential tearing occurs.
A further drawback of the devices of known type is the possibility of the web sliding or slipping on the feed rollers, with consequent loss of the correct phase setting between the cutting means and web.
In such an event, it is often necessary to halt the device and manually adjust in order to reset the correct operating conditions. If it occurs at the commencement of the drive stage, the cause of said slippage is mainly due to the variations in the tension to which the web is subjected.
However, other causes include high operating speeds, wear of the feed roller surfaces and slackness arising between these rollers and their support means.